The invention generally relates to inside-out nuclear magnetic resonance (NMR) measurements, and more particularly, the invention relates to detecting tool motion effects on NMR measurements of formation properties surrounding a borehole, such as measurements of the hydrogen content of the formation, for example.
Nuclear magnetic resonance (NMR) measurements may be used to determine properties of a sample, such as body tissue (for medical imaging purposes) or a subterranean formation (for well logging purposes). For example, for the subterranean formation, NMR measurements may be used to determine and map the porosity, formation type, permeability and oil content of the formation.
Referring to FIG. 1, as an example, NMR may be used in a logging while drilling (LWD) operation to map the properties of a subterranean formation 10. In this manner, an axisymmetric NMR tool 6 may be part of a drill string 5 that is used to drill a borehole 3 in the formation 10. The tool 6 may be, as examples, one of the tools described in Sezginer et. al., U.S. Pat. No. 5,705,927, entitled, xe2x80x9cPulsed Nuclear Magnetism Tool For Formation Evaluation While Drilling Including a Shortened or Truncated CPMG Sequence,xe2x80x9d granted Jan. 6, 1998; Miller, U.S. Pat. No. 5,280,243, entitled, xe2x80x9cSystem For Logging a Well During the Drilling Thereof,xe2x80x9d granted Jan. 18, 1994; Taicher et. al., U.S. Pat. No. 5,757,186, entitled, xe2x80x9cNuclear Magnetic Resonance Well Logging Apparatus and Method Adapted for Measurement-While-Drilling,xe2x80x9d granted May 26, 1998; Jackson et. al., U.S. Pat. No. 4,350,955, entitled, xe2x80x9cMagnetic Resonance Apparatus,xe2x80x9d granted Sep. 21, 1982; or U.S. patent application Ser. No. 09,186,950, entitled, xe2x80x9cApparatus and Method for Obtaining a Nuclear Magnetic Resonance Measurement While Drilling,xe2x80x9d filed on Nov. 5, 1998.
The NMR measuring process is separated by two distinct features from most other downhole formation measurements. First, the NMR signal from the formation comes from a small resonance volume, such a generally thin resonance shell, or volume 20a (see FIG. 2), and the resonance volume 20a may have a radial thickness that is proportional to the magnitude of an oscillating magnetic field and inversely proportional to the gradient of a static magnetic field. Depending on the shape of the resonance zones, the volume may extend, as an example, from as little as 1 millimeter (mm.) in one direction and as long as several inches in another. Secondly, the NMR measurement may not be instantaneous. Both of these facts combined make the NMR measurements prone to tool motions, such as the motion that is attributable to the movement of the NMR tool 6 around the periphery of the borehole 3, as further described below.
To perform the NMR measurements, the NMR tool 6 may include one or more permanent magnets to establish a static magnetic field called B0; a radio frequency (RF) coil, or antenna, to radiate the time varying magnetic B1 field that is perpendicular to the B0 field and an RF coil, or antenna, to receive spin echoes from the formation in response to an NMR measurement, as described below. These two coils may be combined into a single transmit/receive antenna.
As an example, the NMR tool 6 may measure T2 spin-spin relaxation times of hydrogen nuclei of the formation 10 by radiating NMR detection sequences to cause the nuclei to produce spin echoes. The spin echoes, in turn, may be analyzed to produce a distribution of T2 times, and the properties of the formation may be obtained from this distribution. For example, one such NMR detection sequence is a Carr-Purcell-Meiboom-Gill (CPMG) sequence 15 that is depicted in FIG. 4. By applying the sequence 15, a distribution of T2 times may be obtained, and this distribution may be used to determine and map the properties of the formation 10.
A technique that uses CPMG sequences 15 to measure the T2 times may include the following steps. In the first step, the NMR tool 6 pulses the B1 field for an appropriate time interval to apply a 90xc2x0 excitation pulse 14a to rotate the spins of hydrogen nuclei that are initially aligned along the direction of the B0 field. Although not shown in detail, each pulse is effectively an envelope, or burst, of a radio frequency RF carrier signal. When the spins are rotated around B1 away from the direction of the B0 field, the spins immediately begin to precess around B0. The pulse is stopped when the spins are rotated by 90xc2x0 into the plane perpendicular to the B0 field. They continue to precess in this plane first in unison, then gradually losing synchronization. For step two, at a fixed time TCP following the excitation pulse 14a, the NMR tool 6 pulses the B0 field for a longer period of time (than the excitation pulse 14a) to apply an NMR refocusing pulse 14b to rotate the precessing spins through an angle of 180xc2x0 with the carrier phase shifted by xc2x190xc2x0. The NMR pulse 14b causes the spins to resynchronize and radiate an associated spin echo signal 16 (see FIG. 5) that peaks at a time called TCP after the 180xc2x0 refocusing NMR pulse 14b. Step two may be repeated xe2x80x9ckxe2x80x9d times (where xe2x80x9ckxe2x80x9d is called the number of echoes and may assume a value anywhere from several to as many as several thousand, as an example) at the interval of 2.TCP. For step three, after completing the spin-echo sequence, a waiting period (usually called a wait time) is required to allow the spins to return to equilibrium along the B0 field before starting the next CPMG sequence 15 to collect another set of spin echo signals. The decay of each set of spin echoes is observed and used to derive the T2 distribution.
The T2 time characterizes a time for the spins to lose irreversibly their unison precession after the application of the 90xc2x0 excitation pulse 14a. In this manner, at the end of the 90xc2x0 excitation pulse 14a, all the spins are pointed in a common direction that is perpendicular to the static B0 field, and the spins precess at a resonance frequency called the Larmor frequency for a perfectly homogeneous B0 field. The Larmor frequency xcfx89L may be described by the equation xcfx89L=xcex3B0, where xcex3 is the gyromagnetic ratio of the nuclei under investigation. However, the B0 field is not really homogeneous, and the pulse excites spins roughly over the frequency range |xcex94xcfx89| less than xcex3B1, with xcex94xcfx89=xcex3B0xe2x88x92xcfx89rf being the off resonance frequency and xcfx89rf being the carrier frequency of the RF pulses. So after excitation, the spins de-phase with T2* due to inhomogeneities in the static B0 field. This decay is reversible and is reversed by the refocusing pulses 14b that produce the sin echo signals. In addition, irreversible de-phasing occurs (spin-spin relaxation) and is described by the T2 time constant. This effect creates the decay of successive echo amplitudes according to the T2 time constant. Thus, typically, only spins with T2 greater than  greater than T2* are measured.
As stated above, the distribution of the T2 times may be used to determine the properties of the formation. For example, referring to FIG. 6, the formation may include small pores that contain bound fluid and large pores that contain free, producible fluid. A T2 separation boundary time (called TSEPARATION in FIG. 6) may be used to separate the T2 distribution into two parts: one part including times less than the TSEPARATION time that indicate bound fluids and one part including times greater than the TSEPARATION time that indicate free, producible fluids.
Each T2 time typically is computed by observing the decay of the magnitude of the spin echo signals 16 that are produced by a particular CPMG sequence 15. Unfortunately, the drill string 5 (see FIG. 1) may move too rapidly for the NMR tool 6 to accurately observe this decay. However, the T2 time is correlated with another time constant called a T1 spin-lattice relaxation time. The T1 time characterizes the time for the spins to return to the equilibrium direction. Considering both the T1 and T2 times, each spin may be thought of as moving back toward the equilibrium position in a very tight pitch spiral during the T1 decay. Fortunately, the T1 and T2 times are approximately proportional. As a result, the T2-based measurements may be substituted with T1-based measurements. In fact, the original work on establishing bound fluid cutoffs was done using T1. Those results were then expressed and used commercially in terms of T2.
Polarization-based measurements may use either inversion recovery sequences or saturation recovery sequences. An example of an inversion recovery sequence is described in Kleinberg et. al, U.S. Pat. No. 5,023,551, entitled, xe2x80x9cNuclear Magnetic Resonance Pulse Sequences For Use With Borehole Logging Tools,xe2x80x9d granted Jun. 11, 1991. Under xe2x80x9cinside outxe2x80x9d conditions in conjunction with motion, it may be easier to saturate a region than to invert it completely. Therefore, saturating a region may be preferred.
Referring back to FIG. 2, the T1 times typically are measured using polarization-based measurements instead of the decay-based measurements described above. In this manner, each polarization-based measurement may first include applying a saturation sequence to saturate the spins in a resonance region (such as the cylindrical resonance shell, or volume 20a. as depicted in FIG. 2, for example). Subsequently, a polarization period elapses to allow polarization of the resonance volume 20a to the field. Subsequently, a detection sequence, such as the CPMG sequence, is used to produce spin echoes from the formation 10. The amplitudes of the first few spin echo signals are then analyzed to determine an amplitude. Because only the first few echoes need to be observed to determine the amplitude of the signal, the T1 measurement may be performed in a shorter duration of time than the decay-based T2 measurement and thus, may be less prone to motion of the NMR tool 6. The detection sequence may be successively repeated (after the appropriate saturation sequence) several times with varied wait times to obtain a distribution of T1 times.
As an example, a polarization-based measurement may be used to measure the T1 times for hydrogen nuclei in the resonance volume 20a (see FIG. 2) located within the saturated volume 20b. In this manner, the NMR tool 6 may first saturate spins within the volume 20b. However, the polarization period may be sufficiently long to permit tile NMR tool 6 to significantly move within the borehole and cause the NMR tool 6 to receive spin echo signals from a shifted resonance volume 20axe2x80x2 (see FIG. 3) that partially overlaps the original, saturated volume 20b. As a result, tile shifted resonance volume 20axe2x80x2 may include a region without saturated spins (an effect typically called xe2x80x9cmoving fresh spins inxe2x80x9d) and a region of the original saturated volume 20b with saturated spins. Unfortunately, polarization-based NMR.
One way to identify potential problems caused by motion effects may be to use a motion detection device, such as a strain gauge, an ultrasonic range finder, an accelerometer or a magnetometer. In this manner, the motion detection device may be used to establish a threshold for evaluating the quality of the NMR measurement. Such an arrangement is described in PCT Application Number PCT/US97/23975, entitled, xe2x80x9cMethod for Formation Evaluation While Drilling,xe2x80x9d that was filed on Dec. 29, 1997. However, conventional motion detection devices may not specifically indicate corrections that are needed to be made to the measurement data to compensate for tool motion.
Thus, there is a continuing need for an arrangement to more precisely detect tool motion effects on NMR measurements. There is also a continuing need for an arrangement to more precisely quantify tool motion effects on NMR measurements.
An NMR measurement apparatus is used to perform at least one NMR measurement of a sample. The measurements are used to determine an effect of motion between the measurement apparatus and the sample. In one embodiment of the invention, NMR measurements of the same type but with varied parameters are performed that have different sensitivities to the motion, and the results are compared to determine an effect of the motion. In another embodiment, an NMR measurement is performed to measure spin-spin relaxation times of the sample; another NMR measurement is performed to measure spin-lattice relaxation times; and the results are compared to determine an effect of the motion. In another embodiment, measurements are performed in different regions that are supposed to have different saturation thicknesses, and these measurements are used to determine an effect of the motion. In another embodiment, a characteristic of at least one spin echo signal is analyzed to determine an effect of the motion. In yet another embodiment, NMR measurements are performed in different radially adjacent regions and the results are compared to determine an effect of the motion.
Advantages and other features of the invention will become apparent from the following description, drawing and claims.